1. Field of the Invention
The present invention relates to an elastic wave device used as, for example, a resonator or a band-pass filter, and a method of producing the same. More specifically, the present invention relates to an elastic wave device in which a portion where wiring patterns on a piezoelectric substrate are stacked on each other is improved, and a method of producing the same.
2. Description of the Related Art
Recently, with the reduction in the size of mobile phones, the reduction in the size of a band-pass filter used in mobile phones has also been strongly desired. As such a band-pass filter, a surface acoustic wave device using surface acoustic waves has been widely used. In a surface acoustic wave device, in order to achieve the reduction in the size thereof, a surface acoustic wave filter chip, in which electrically conductive films for forming an interdigital transducer (IDT) and the like are provided on a piezoelectric substrate, is connected to a package not by wire bonding but by flip-chip bonding using bumps.
Japanese Unexamined Patent Application Publication No. 2006-115548 described below discloses an example of such a surface acoustic wave device.
FIG. 23 is a partially cutaway front cross-sectional view illustrating the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2006-115548. A surface acoustic wave device 1001 includes a piezoelectric substrate 1002. An IDT 1003 composed of a multilayer electrically conductive film is provided on the piezoelectric substrate 1002. The IDT 1003 includes a main electrode layer 1003a composed of Cu. An adhesive layer 1003b composed of Ti is stacked under the main electrode layer 1003a. The formation of the adhesive layer 1003b composed of Ti increases the adhesion strength of the IDT 1003 to the piezoelectric substrate 1002.
A protective layer 1003c composed of Al is stacked on the main electrode layer 1003a. Since Al is less easily oxidized than Cu, the main electrode layer 1003a can be protected by the protective layer 1003c. Note that, in the surface acoustic wave device 1001, in order to improve frequency temperature characteristics and to protect the device, the IDT 1003 is covered with a silicon oxide film 1004.
On the other hand, Japanese Unexamined Patent Application Publication No. 2003-174056 described below discloses an IDT including an underlying layer composed of Ti and disposed on a piezoelectric substrate and a main electrode layer composed of Al and disposed on the underlying layer. That is, an IDT having a stacked structure of Al/Ti from the top in that order is disclosed. In addition, in Japanese Unexamined Patent Application Publication No. 2003-174056, an electrode pad that is electrically connected to the IDT includes a lower electrode composed of Al, an upper electrode composed of Al, and a barrier layer composed of Ti and stacked between the lower electrode and the upper electrode. That is, the electrode pad has a stacked structure of Al/Ti/Al from the top in that order. This is so that the formation of cracks of the piezoelectric substrate is prevented during bonding using bumps in flip-chip bonding by forming the lower electrode composed of Al, which is relatively soft, on the piezoelectric substrate.
As described in Japanese Unexamined Patent Application Publication No. 2003-174056, in an electrode pad portion where bump bonding is performed, cracks in the piezoelectric substrate can be prevented by providing the bottom electrode layer composed of Al.
Now, it is assumed that a wiring pattern leading to an IDT of a surface acoustic wave device is referred to as “a first wiring pattern”, and a wiring pattern leading to the above-mentioned electrode pad is referred to as “a second wiring pattern”.
The second wiring pattern leading to the electrode pad is simultaneously formed with the electrode pad. Accordingly, when the electrode pad portion is formed as described in Japanese Unexamined Patent Application Publication No. 2003-174056, the second wiring pattern leading to the electrode pad similarly also has the stacked structure of Al/Ti/Al. In a contact portion where such a second wiring pattern is overlapped with the first wiring pattern leading to an IDT to establish an electrical connection, the Al film, which is the bottom electrically conductive film of the second wiring pattern, is overlapped with the top electrically conductive film of the first wiring pattern.
Accordingly, as described in Japanese Unexamined Patent Application Publication No. 2006-115548, when the multilayer electrically conductive film constituting the IDT has a stacked structure of Al/Cu/Ti, the first wiring pattern also has the same stacked structure. Accordingly, in the contact portion, an Al film and an Al film are overlapped with each other. In such a structure, the contact resistance in the contact portion increases, and thus the insertion loss of the surface acoustic wave device tends to degrade.